Transducers which are used to measure dynamic motion or force exist in extremely large variety, but virtually all consist of two general components: a mechanical structure designed such that the applied motion or force input causes an internal deflection proportional to that input, and an electrical element which measures that deflection in a manner that the electrical output of the sensor is proportional to the magnitude of the deflection. The transduction from deflection to an electrical parameter defines the generic term “transducers” for such devices.
The form of electrical output of a dynamic sensor can vary to include virtually any parameter that is electrically measurable. The most common measurement is the number of electrons (charge) displaced by a dynamic event; however, the measurement might also be the electric field strength (voltage) that caused those electrons to be displaced, or the ratio of charge to voltage (capacitance), the rate of electron flow (current), the ratio of voltage to current, (impedance or resistance), etc. As an example, the basic design of an accelerometer consists of a thin plate of piezoelectric (PE) material clamped between a “base” and a “mass”, a piece of material which serves as an inertial component. When the mounting surface to which the base is attached is accelerated, the acceleration of that inertial mass will result in a force which causes the PE material to deform. The PE property will induce electrons to gather on one side of the plate. Electrodes attached to the appropriate surfaces of the plate lead to the electrical cable of the transducer.
Since electrons can be induced from a myriad of external sources, such as electrical fields or perhaps by the rubbing of insulators in the electrical wires delivering the output to the data acquisition, the prior art teaches specialized electronics and shielding techniques to reduce the errors caused by such external noise sources. One such technique is a circuit to perform an impedance conversion, internal to the sensor, which translates the quantity of electrons to a voltage level. The circuitry floods the output with sufficient electrons to maintain the voltage level, so that undesirable electrons induced by external fields and rubbing insulators do not significantly affect the overall output. These circuits are made by multiple manufacturers, under many trademarks (ICP®, ISOTRON®, DELTATRON®, etc), but all can be lumped under the term IEPE (for Internal Electronic PiezoElectric).
IEPE uses two wires: output and ground. The source of power for an IEPE device is a constant current. The current for a prior art IEPE circuit is not well regulated, which is adequate for use with a PE bridge sensor. The output of the IEPE device, which represents the time-varying dynamic input to the transducer, takes the form of an analogously time-varying voltage component added to the static voltage operating point of the IEPE circuit. Such circuitry has been extensively used in the industry, due to its advantages in reduced noise, reduced cable costs, simplicity of the associated external conditioning, etc.
One characteristic of IEPE devices is that the static voltage operation point does not vary analogously to the parameter to be measured, even if that parameter had significant static value. In contrast, there is another class of transducers the output of which does vary analogously with the static or low frequency inputs being measured. Such static measurements result in a direct current or DC output proportional to the static parameter to be measured. This DC coupling is in contrast to the alternating current or AC coupling characteristic of circuits such as IEPE, which provide no measurement information at static or low frequencies.
MEMS (Micro Electro Mechanical Systems) are DC coupled sensors used in accelerometers, particularly accelerometers incorporating a Wheatstone bridge of piezoresistive (PR) bridge strain gauges. These devices are used to measure static or low frequency values, and in addition are more successfully used in certain applications such as violent shock. More traditional PE transducers can respond with erroneous output caused by the event. An example of an error is a step change in output occurring during the most violent period of a shock (called “zero shift”). MEMS PR devices tend to perform well in such applications, not displaying such zero shift. The prior art teaches the use of a bridge sensor signal conditioner, external to the transducer, to address the requirements of Wheatstone bridges, for example, to provide a carefully controlled excitation voltage or current and specialized circuits to handle the common mode voltage and differential output of the bridge. A bridge sensor signal conditioner includes a highly regulated and therefore costly excitation voltage or current source, circuitry to vary the excitation voltage or current, and circuitry to subsequently adjust other parameters affected by the varying excitation voltage or current, such as gain. As a result, a bridge sensor signal conditioner is more complex and costly to manufacture and physically larger than the IEPE circuitry noted supra and more difficult and complicated to operate.
Thus, the prior art teaches that to use a bridge sensor signal conditioner with a sensor, the excitation voltage or current of the conditioner must be selected by the user, the parameters, such as gain, noted above, must subsequently be adjusted by the user, and trial runs made by the user to ensure that the various selections and adjustments have been properly made. Thus, the prior art also teaches against modularity of bridge sensor signal conditioners. For example, once a conditioner has been adjusted and tested for compatibility with a specific sensor, the conditioner cannot be used with another sensor, even a sensor of the same general type, without repeating the adjustment and testing noted above with respect to the other sensor.
FIG. 1 is a block diagram of a prior art bridge sensor transducer. As previously described, a PR sensor consists of a material that changes its electrical resistance as a function of the quantity to be measured—temperature, force, strain, etc. Such sensors are normally configured as a four legged Wheatstone bridge to maximize the output. FIG. 1 shows a typical sensor in a known Wheatstone bridge arrangement. Resistors Ra, Rb, Rc, and Rd are typically embodied in MEMS devices. The prior art teaches the use of bridge sensor signal conditioner and six lines connecting the bridge sensor to the bridge sensor signal conditioner. The excitation lines are used to provide power to the bridge sensor and the sense lines are used to address voltage regulation problems, for example, compensation for losses due to line lead length. The data acquisition signal (DAQ), the output of the bridge sensor signal conditioner bridge sensor signal conditioner, is analogous to an output of the IEPE device described supra. As noted above, the prior art teaches the use of a precise/well-regulated voltage or current source for the excitation lines and a series of adjustments and tests.